Embodiments of the present invention relate to a semiconductor device and a method for fabricating the same, and more particularly to a technology for depositing a nitride material having a lower etch rate than an oxide material over or between buried gates when forming a metal contact at an end portion of a cell region.
Although the demand of implementing high-capacity dynamic random access memory (DRAM) is rapidly increasing, there is difficulty in increasing chip size, resulting in a limitation in increasing storage capacity of DRAM. The larger the chip size, the less the number of chips on each wafer, resulting in a reduction of productivity. Therefore intensive research is being conducted into a variety of methods for reducing a cell region by varying a cell layout so as to form a large number of memory cells on one wafer.
A buried gate structure has been developed as an example of the above-mentioned methods. The buried gate is located below a semiconductor silicon substrate and a metal contact for voltage supply (or power supply) to the buried gate is required.
Usually, a metal contact transmits input/output (I/O) operation signals to the buried gate is formed in a larger size than that of underlying buried gate.
Thus, if misalignment occurs in a masking process for metal contact formation, a device isolation film between the buried gates (BGs) in a cell region is attacked. Furthermore, a semiconductor substrate (Sub) below the device isolation film also might be attacked. It causes current leakage or an electrical short, resulting in reduction of reliability of the semiconductor device.
In order to solve the above-mentioned problems, an over-etch process in the cell region should be refrained from so that the metal contact in the cell region is formed more shallow. For example, a bottom of the metal contact can be formed at a higher level than any surface of the substrates 101a, 101b, 101c. However, a depth of the metal contact in the cell region cannot be arbitrarily adjustable because another metal contact is formed, in a simultaneous process, in a peripheral region so as to extend down to a substrate in the peripheral region.